In recent years, as a means for coping with regulations of exhaust from diesel engines, the development of injection nozzles having smaller nozzle orifice diameters and injection pumps having higher pumping pressure has been promoted to atomize injected fuel more finely. Formerly, in many cases, the startability at a low temperature could be improved by injecting more fuel by means of a starting increase function or the like. However, atomization of fuel promotes vaporization of fuel during the ignition lag period. Therefore, if the amount of injected fuel is excessively large, the fuel takes a large amount of heat of vaporization, reducing the temperature in the engine so that misfire can occur as an effect contrary to the original purpose. There is, therefore, a need for more accurate control of the fuel injection rate.
For example, in an in-line fuel injection pump or the like for diesel engines, the position of a control rack for injection rate adjustment (hereinafter referred to as "rack" or "control rack") of a governor is controlled to change the rate of injection of fuel into the engine. However, even when the position of the rack is fixed, the fuel injection rate is not constant with respect to the engine rotating speed. That is, an ordinary characteristic of an injection pump is such that, as shown in FIG. 6, the fuel injection rate decreases if the engine rotating speed becomes lower when the rack position is fixed. This tendency is stronger in a very low engine speed range.
On the other hand, to improve the startability of an engine, it is necessary to supply the engine with fuel at a rate high enough to achieve such an engine rotating speed that the engine can rotate against resistance. In a diesel engine using a mechanical governor, therefore, the fuel injection rate is increased in a lower rotating speed range at the time of starting the engine relative to the fuel injection rate at the time of ordinary control. In this method, a starting increase mode is set in which the rack position at the time of starting is on the fuel injection rate increasing side of the rack position at the time of ordinary control, as indicated by the hatched area in FIG. 7, thereby increasing the fuel injection rate at the time of starting.
This starting increase is effected by using a spring which is called a start spring and which is comparatively weak in tensile force. Between A and B, after an engine start and before the rotating speed becomes equal to N.sub.1, the rack position is constantly maintained stationary, by being stopped by a stop mechanism such as a rack cap. The rack position between B and C, from the rotating speed N.sub.1 to the rotating speed N.sub.2, is determined by the balance between the tensile force of the start spring and the centrifugal force of a fly weight of the governor. However, since the spring force is weak, the rack position changes abruptly to its ordinary control position, by moving in the injection rate reducing direction with a steep gradient to reduce the amount of starting increase to zero.
There is another method using an electronic governor for electronically controlling the amount of operation of an actuator for operating the control rack to adjust the fuel injection rate. Also in this case, a control mode of increasing the injection rate at the time of starting, as shown in FIG. 8, is adopted. In this starting increase mode, the starting increase action of the mechanical governor is directly replaced with the electronic governor, as is apparent from the figure. That is, between a and b, before the engine rotating speed becomes equal to N.sub.1, the rack position is constantly maintained stationary so that the amount of increase is X.sub.0. The rack position is rapidly adjusted between b and c, in which the rotating speed changes from N.sub.1 to N.sub.2, and the amount of increase is thereby set to zero at N.sub.2. Such setting is made in almost all cases. The change between b and c of FIG. 8 is called regulation R and is expressed by EQU R={(N.sub.2 -N.sub.1)/N.sub.1 }.times.100(%) (1)
The value of the regulation R is ordinarily 20% or less.
However, considering the points described below, it cannot be said that the setting of the above-described conventional starting increase mode is optimal and effective in sufficiently improving the startability. In the conventional starting increase mode, as shown in FIG. 7 or 8, the rack position is constantly maintained stationary between A and B or between a and b. However, in a very low speed range of the engine rotating speed, the fuel injection rate increases abruptly as the rotating speed becomes higher even when the rack position is constant, as shown in FIG. 6. For this reason, even if the rack position is set for an optimal injection rate at the point A or a at the beginning of the starting time, fuel is necessarily injected at an excessively high rate at the point B or b, at which the engine rotating speed becomes equal to N.sub.1. Thus, the conventional starting increase mode entails the problem of occurrence of misfire due to an increase in the heat of vaporization taken by the fuel. Further, between B and C or between b and c, the rack position is rapidly returned to the ordinary control position, so that the fuel injection rate decreases abruptly. Therefore, when the resistance to rotation is large, for example, in the case where the temperature of the engine is low, a situation can occur easily wherein the rotating speed cannot be increased as desired because fuel is not sufficiently supplied.